The immunochromatographic assay known as a “rapid testing method” has been developed for a simple, qualitative and quantitative analysis of a small amount of analyte. The assay utilizes antigen-antibody reaction for analysis, and is applied in various fields or industries, such as a medical field for diagnosing diseases, agriculture, livestock raising, foods, military, environment and so on. Typically, the immunochromatographic assay is carried out with an assay strip, or with an assay device which comprises a plastic case and the assay strip installed in the case. FIG. 1 is a cross-sectional view of the conventional assay strip for immunochromatographic assay. As shown in the FIG. 1, the conventional assay strip 10 includes a sample pad 12 for receiving a liquid sample, a conjugate pad 14 containing a conjugate, a porous membrane pad 16 immobilized with a binder (antibody or antigen) 16a which specifically combines with an analyte in the sample and/or the conjugate, and an absorbent pad 18 for finally receiving the liquid sample. The conjugate is produced by conjugating a label which generates a signal detectable by a naked eye or a sensor to a ligand such as an antigen or an antibody. These pads are consecutively arranged on a plastic backing 20, and the connecting ends of the neighboring pads are overlapped by a predetermined distance. In case the assay strip 10 is installed in a plastic case to form the assay device, a sample receiving hole for introducing a sample to the sample pad 12, and a result observation window for observing the test result are formed on the upper portion of the case.
In the immunochromatographic assay using the assay strip 10, a liquid sample is injected into the sample pad 12. The injected liquid sample flows along the conjugate pad 14 and the porous membrane pad 16 by a capillary action, and is finally absorbed by the absorbent pad 18. In this case, the conjugate in the conjugate pad 14 also migrates along with the liquid sample. If a target analyte exists in the sample, the conjugate is bound to the binder 16a which is immobilized on the porous membrane pad 16 via the target analyte (i.e., the target analyte intervenes between the conjugate and the binder 16a. “sandwich reaction”), or the conjugate and the target analyte are competitively bound to the binder 16a (“competition reaction”). Therefore, the existence of the analyte can be determined by detecting the conjugate bound to the binder 16a with a naked eye or a sensor. However, in order to control the flow rate of the mobile phase, such as the liquid sample, the pore size of the porous membrane pad 16 should be modified. Thus, the flow speed of the liquid sample and the reaction rate of antigen-antibody reaction cannot be effectively controlled with the conventional assay strip 10. Namely, the dilution of the liquid sample with a diluent, and/or the antigen-antibody reaction(s) between the binder 16a, the analyte in the liquid sample and/or the conjugate cannot be carried out for enough time interval, which deteriorates the sensitivity and specificity of the immunochromatographic assay. In addition, when a pad, for example the porous membrane pad 16, is damaged during the production of the assay strip 10, the whole assay strip 10 should be discarded because the pads are serially connected. The conventional assay strip 10 has a further disadvantage in that the shape of the assay strip 10 or the shape of the assay device including the strip 10 is limited to the straight rod shape.
In addition, when whole blood is used as the liquid sample, the flow speeds of the whole blood components, such as the hemolyzed erythrocyte, are relatively slow compared to the analyte in the liquid sample. Namely, the whole blood components, such as the hemolyzed erythrocyte, pass through the sample pad 12 and the conjugate pad 14 having pores of small sizes with relatively slow speed, and remain in the porous membrane pad 16 for a long time. Therefore, when the desired antigen-antibody reaction is carried out on the detection line on which the binder 16a is immobilized, the whole blood components still exist on the porous membrane pad 16, namely, on the background, and the color of the detection line is covered with the red color of the whole blood components. This makes it difficult to determine the test result. Therefore, the test result should be determined after the whole blood components completely pass the detection line. However, this is not practical since it takes much time for the detection or determination of the test result. Such problems mainly occur when using the whole blood to examine disease, such as malaria, AIDS, hepatitis C, hepatitis B, syphilis, helicobacter pylori, tumor markers (AFP, PSA, CEA), tuberculosis, SARS, Dengue fever, leprosy, and so on.